Plug screw and Bivis treatment under pressurized conditions and subsequent LC refining

Unbleached kraft pulp for making sack paper is usually first refined in a high consistency process followed by low consistency refining in order to achieve desired paper properties, such as high strain at break and high tensile energy absorption at low airflow resistance. Conventional atmospheric high consistency refining is compared to treatments using two types of compressive screw equipment, Bivis equipment and the pressure expansion (PREX) plug screw feeder, commonly used in thermo mechanical pulp (TMP) plants. The plug screw pretreatments were performed under atmospheric and steam-pressurised conditions at 120°C and 130°C. Treatment in the Bivis equipment was performed under different compressive and shear conditions. After the Bivis and plug screw treatments, the pulp samples were refined in a low consistency process using the Escher-Wyss refiner. Paper properties were evaluated on freely dried sheets which were produced in order to simulate paper made under industrial sack paper drying conditions. The highest levels of the tensile energy absorption index and strain at break were achieved from curled and kinked fibres, pre-treated under the most compressive and highest shear stress conditions before low consistency refining. These results were obtained at very low airflow resistance, which is advantageous for sack paper, and with very low energy consumption during pre-treatment with the plug screw. The pulp samples pre-treated at high consistency in a plug screw feeder under steam-pressurised conditions or in the Bivis equipment showed, after low consistency refining, increased shrinking ability compared to the pulp samples pre-treated in high consistency refining under atmospheric conditions.